Filter system

ABSTRACT

The present invention is an improved filtration system, filtering method and unique chemical composition for capturing mercury and other pollutants in flue gases generated by process gas streams. The improved filtration system may take various forms depending on the type of filter system most desired for a particular application; however, the filter system includes at least a filter element or elements and an adsorbent component having a composition suitable for capturing mercury on the downstream side of the filter element.

RELATED APPLICATIONS

The present application is a division of copending U.S. patentapplication Ser. No. 10/272,487 filed Oct. 16, 2002.

FIELD OF THE INVENTION

The present invention is an improved filter system and componentsthereof for both collecting particulates and adsorbing chemicalpollutants from process gas streams. More particularly, the filtersystem of the present invention is highly effective for removingpollutants such as gaseous and solid elemental mercury and itscompounds, as well as other chemical pollutants, and for collectingparticulates under a range of process gas stream conditions.

BACKGROUND OF THE INVENTION

The removal of particulates from a gas stream has long been a practicein a variety of industrial fields. Conventional means for filteringparticulates and the like from gas streams include, but are not limitedto, filter bags, filter tubes, filter cartridges and filter panels.These filter elements are typically oriented into a filtration system,often referred to as a filter baghouse, for filtering such particulates.Such filtration systems may be either cleanable or non-cleanable,depending on the requirements of the system operation. Referring to FIG.1, there is shown one non-limiting example of a conventional filtrationsystem, in this case a pulse jet cleaning system and sequence is shown.Inside hopper 120, the particulate laden gas stream 121 enters thehopper at inlet 122 and passes through filter bag 123. Tube sheet 125inside hopper 120 prevents the gas stream from bypassing the filter bag.The filter bag 123 is kept open by support cage 126. The gas stream,after passing through the bag and out bag exit 129, exits the clean aircompartment at outlet 127. In operation, particulate forms a dust cake128 on the outside of the filter bag, as shown in the bag on the left ofthe figure. On cleaning to remove the filter cake, air from pulse pipe130 enters the bag. This pulse of air 132 expands the bag, loosening thedust cake and thus causing particulate 131 to collect at the bottom ofthe hopper 120. As seen in the bag on the right of the figure, the pulsejet causes the filter bag to expand.

Activated carbon powders have been used for the capture of toxiccontaminants such as mercury in flue gas streams emanating from utilityboilers, hazardous or municipal waste incinerators, crematoria and thelike. Typically, activated carbon powder is fed, or “injected,” into aflue gas stream upstream of a particulate collection device. One exampleof such an activated carbon capture system is described in the articleentitled Full-Scale Evaluation of Sorbent Injection for Mercury Controlon Coal Fired Power Plants by Bustard et al, In Proceedings of AirQuality III: Mercury, Trace Elements and Particulate Matter Conference;Arlington, Va., Sep. 9-12, 2002. This publication teaches that activatedcarbon powder, can be introduced upstream of a filter bag dust collector(e.g., a baghouse) to adsorb or react with the mercury in the gasstream, then the adsorbed or reacted mercury is collected on the surfaceof the filter bag or bags.

The problem of the capture and immobilization of gaseous mercury and itscompounds has been considered previously. For example, continuousinjection of powdered activated carbon (PAC) into the flue-gas trainupstream of an electrostatic precipitator or fabric filter has been usedto control mercury emissions in the municipal waste combustor industryand has been proposed as a control technology for the coal-fired utilityindustry in the United States. [John H. Pavlish et. al., “Status Reviewof Mercury Control Options for Coal-Fired Power Plants”, Fuel ProcessingTechnology, in press (2002), and references therein] Disadvantages ofthis approach include the need for large volumes of carbon to adsorbmercury to proposed regulatory levels due to the short contact time ofthe adsorbent carbon in contact with mercury vapor and the low capacityfor mercury adsorption by PAC at temperatures above about 130° C. Inaddition, the requisite C/Hg injection ratios necessitate large volumesof injected carbon that can result in considerable contamination of thefly ash produced in coal-fired utility boilers. Carbon contaminationoften reduces the commercial value of the ash as an additive forconcrete.

Various treatments of PAC with sulfur compounds or elemental iodine toimprove equilibrium adsorption capacity or capture efficiency formercury have been investigated and disclosed. The better performersamong these known in the art have been summarized by Pavlish, referencedabove. For example, U.S. Pat. No. 3,876,393 discloses the passing ofmercury-containing vapors over activated carbon that has beenimpregnated with sulfuric acid. Unexamined Japanese Patent Application(Kokai) No. 10-109016 (Babcock Hitachi KK) teaches that activated carbonpowder, or another component having a large specific surface, treatedwith a ZnI₂ active component can be introduced via a carbon injectionsystem to remove mercury in a flue gas stream.

Disadvantages associated with the use of such systems include expensiveinjection systems, limited collection efficiencies, particularly at hightemperatures (i.e., above 130° C.), and carbon-contaminated fly ash thatmay require handling as hazardous waste. Initial testing with a PowderRiver Basin (PRB) ash determined that the presence of even trace amountsof activated carbon in the recovered ash rendered the materialunacceptable for use in concrete (Bustard et al).

The use of activated carbon fiber filters for mercury capture isdescribed in Journal of the Air & Waste Management Association, Vol. 50,June 2000, pages 922-929. It is taught that activated carbon fibers canbe woven or felted and used in a bag filter configuration whereparticulate matter and elemental mercury could be captured. However, theadsorptive capacity reported in this work (52.5 micrograms Hg/gramactivated carbon) is too low to allow this to be used as a bag filter orfixed bed in place of carbon injection. Furthermore, a bag filter madefrom activated carbon fibers will trap fly ash particles within thedepth of the fiber structure, causing a steep increase in pressure dropover time, and the cleanability of such bags is very limited.

It is known to incorporate catalytic and adsorbent particles into filterelements to react with and/or adsorb components from a gas stream. InU.S. Pat. No. 4,220,633 and U.S. Pat. No. 4,309,386, to Pirsh, filterbags are coated with a suitable catalyst to facilitate the catalyticreduction process of NO_(x). In U.S. Pat. No. 5,051,391, to Tomisawa etal., a catalyst filter is disclosed which is characterized in thatcatalyst particles which are made of metal oxides with a diameter ofbetween 0.01 to 1 um are carried by a filter and/or a catalyst fiber. InU.S. Pat. No. 4,732,879, to Kalinowski et al., a method is described inwhich porous, preferably catalytically active, metal oxide coatings areapplied to relatively non-porous substrates in a fibrous form. In patentDE 3,633,214 A1, to Ranly, catalyst powder is incorporated intomultilayered filter bags by inserting the catalyst into the layers ofthe filter material. Further examples to produce catalytic filterdevices include those set forth in JP 8-196830, to Fujita et al., inwhich a micropowder of an adsorbent, reactant, or the like is supportedin a filter layer interior. In JP 4-219124, to Sakanaya et al., acompact, thick, and highly breathable filter cloth is filled withcatalyst for the bag filter material in order to prevent catalystseparation. In U.S. Pat. No. 5,620,669, to Plinke et al., the filtercomprises composite fibers of expanded polytetrafluoroethylene (ePTFE)having a node and fibril structure, wherein catalyst particles aretethered within the structure. PCT Publication No. PCT/US00/25776, inthe name of Waters et al., is directed to filters comprising activeparticles that are adhered to a porous woven or non-woven substrate by apolymer adhesive, and optionally adjacent or within the substrate is atleast one protective microporous layer. However, none of thesereferences discloses or suggests the removal of mercury from a flue gasstream or appropriate chemical composition for effective mercuryremoval. Moreover, none of these references teaches the removal ofparticulate and mercury contaminants from a flue gas stream wherein thefly ash particulate is collected at a first location and the mercury iscollected downstream of the particulate to minimize or prevent carboncontamination of the particulate fly ash.

An approach to oxidize mercury catalytically to an ionic form that couldbe removed in a subsequent, downstream wet-flue-gas-desulfurization unitoperation (WFGD) was reported by Blythe et al. in “Catalytic Oxidationof Mercury in Flue Gas for Enhanced Removal in Wet FGD Systems”.Promising catalysts were evaluated in packed bed configurations fortheir abilities to generate a soluble mercury species. The authorsanticipated the eventual incorporation of these catalysts into ahoneycomb catalytic oxidizer located in the flue-gas train of coal-firedutility boilers immediately after an ESP dust removal unit. Blythe et.al. expected that locating the catalytic oxidizer monolith after the ESPwould alleviate high pressure drop caused by fly ash plugging, as hadbeen a concern during their packed bed tests. Although they envisionedthe removal of mercury downstream of the point of oxidation and of flyash removal, their concept requires the installation of a separate unitoperation facility with a separate footprint. Furthermore, mercury isrecovered in a relatively dilute liquid phase that might require furthertreatment to concentrate the mercury.

A need clearly exists for an improved filtration system whicheffectively removes mercury in any oxidation state from flue gases atelevated process temperatures (i.e., >130° C.) without the creation ofvoluminous byproducts or waste streams. In addition, a need exists forsuch a system which could be readily retrofit into existing filtersystems without significant and expensive modifications to such existingfilter systems. A further need exists for a mercury filtration systemwhich provides extended on-line operational capability and lessmaintenance compared to at least carbon injection systems. Anotherimportant need exists for the capability of a single filtration systemto provide multi-pollutant control (i.e. particulate, NOx, dioxins,furans, and mercury).

These and other purposes of the present invention will become evidentbased upon a review of the following specification.

SUMMARY OF THE INVENTION

The present invention is an improved filtration system, filtering methodand unique chemical composition for capturing mercury and otherpollutants in flue gases generated by process gas streams. The improvedfiltration system may take various forms depending on the type of filtersystem most desired for a particular application; however, the inventionincludes at least a filter element or elements and an adsorbentcomponent having a composition suitable for capturing mercury on thedownstream side of the filter element.

Filter elements and systems suitable to the present invention includeeither conventional (i.e., non-membrane) or membrane filter bags,cartridges, panels, and the like, whether in a baghouse or otherfiltration assembly.

The adsorbent component of the invention is one with a unique chemicalcomposition which is capable of capturing mercury (Hg) under theoperating conditions of the filter system employed. In a preferredembodiment, the adsorbent component provides a mercury capture capacityexceeding at least 4 mg Hg per gram, more preferably exceeding at least10 mg Hg per gram, and most preferably exceeding at least 20 mg Hg pergram of initial dry weight of adsorbent as determined by the MercuryCapture Efficiency and Capacity Test, described in more detail herein.The adsorbent component comprises an air-permeable high surface areasupport, a mercury binding agent and a promoter. Suitable high surfacearea support materials are those which display a BET (Brunner EmmettTeller) surface area of at least about 50 m²g⁻¹, more preferably atleast 85 m²g⁻¹, and most preferably at least 300 m²g⁻¹ using nitrogen asadsorbate after pretreatment between 100-200° C. for at least 2 hoursunder dynamic vacuum. Activated carbon is just one example of anappropriate support material.

Suitable mercury binding agents may include anions or salts selectedfrom halides, thiocyanates, sulfides, polysulfides, selenium, telluriumand phosphorus compounds Particularly, the binding agent comprising atleast one binding compound having a component selected from the groupconsisting of an anion of a halide, an anion of a thiocyanate, an anionof a sulfide, an anion of a polysulfide, an anion of selenium, anoxyanion of selenium, an anion of tellurium, an oxyanion of tellurium,an anion of phosphorus, and an oxyanion of phosphorus. Preferred bindingagents are selected from the group of potassium iodide, copper iodide,zinc iodide and copper thiocyanate.

The promoter functions to increase adsorption capacity (MCC40 accordingto the test defined herein), to reduce volatility of binding agents, andto generate anchoring sites on the high surface area support for bondingto the mercury-binding-agent complex. Suitable promoters includecompounds having at least one of a multivalent metallic cation and amultivalent metalloid cation that is not spontaneously reduced byiodide, i.e. with a standard aqueous reduction potential Emf value at25° C. less than (more negative than) −0.56 volts, when expressed as atwo electron half-reaction written as a reduction as tabulated in TheCRC Handbook of Chemistry and Physics, 67^(th) edition, R. C. Weast,ed., CRC Press, Boca Raton, 1986, pp. D-151-D-158. Preferred promoterscomprise carboxylate salts of Zn (II) or Mg (II). A particularlypreferred combination of binding agent and promoter comprises potassiumiodide as the binding agent and zinc acetate hydrate as the promoter.Trivalent or quadrivalent metallic or metalloid cations that meet thereduction potential criterion above are suitable for use. It isunderstood that after exposure to heating during preparation or use ofthe mercury-trapping filter, the nature of the binding agent or thepromoter compounds may be altered. The acceptable or optimal ratios ofbinding agent to promoter vary depending on the specific combinations ofingredients used, but follow these general guidelines: acceptablecompositions include molar ratios of mercury-binding-anion topromoter-cation ranging from 1000 to 0.05. Preferred molar ratios rangefrom about 100 to about 2. Most preferred molar ratios range from about5 to about 3.

The improved filtration system of the present invention, in a firstembodiment, comprises a filtration system including one or moreconventional or membrane filter elements (e.g., bags, cartridges, etc.)which incorporate a permanent or replaceable adsorbent insert on thedownstream side of the filter element. Particulate filtration occurs onthe upstream side of the filter element, and mercury removal occurs onthe downstream of the filter element as the flue gas passes through theinsert. The adsorbent insert comprises a suitable air-permeable highsurface area support having thereon a mercury binder/promoter asdescribed above. The adsorbent insert of the invention may be flexibleor rigid. Examples of flexible inserts include woven or felted materialsimbibed with activated carbon particles having the mercury bindingagent/promoter thereon or even activated carbon fibers woven or feltedinto a flexible sheet having the mercury binding agent/promoter thereon.An example of a suitable rigid activated carbon insert material is acarbonized resin, such as that described in U.S. Pat. No. 5,834,114, toEconomy et al. The insert may have any desired geometry such as a flatdisk or panel, a sleeve or tube, a hub-and-spoke geometry, canister orthe like, provided the insert fits into the filter element or is somehowattached to the downstream side of the filter element. The airpermeability of an adsorbent insert having thereon a mercurybinder/promoter should preferably have a Frazier number greater than 20,more preferably greater than 30 and most preferably greater than 40. Inthe filter system where the activated carbon insert component isinstalled inside a filter element, the air permeability of the combinedlayers should have a Frazier number greater than 1, preferably greaterthan 2. Depending on the desired pollutants to be adsorbed, multipleinserts, or inserts with multiple layers may be incorporated in theimproved filter systems of the present invention. A particular benefitof such an insert construction is that the insert may be easily insertedinto or attached to conventional or membrane filter elements to allowretrofitting of existing filtration systems.

In a second embodiment of the invention, the adsorbent component islocated in a layer of the filter element, such as in the filter layeror, more preferably, in a backer layer of the filter element. In thiscase, the flue gas to be filtered contacts the upstream side of thefilter element and particulate filtration is achieved, either by depthfiltration for conventional (i.e., non-membrane) filter elements or bysurface filtration for membrane filter elements. The mercury-containingflue gas, which has been filtered of particulate, then contacts theadsorbent component of the filter element to achieve mercury removalfrom the flue gas. The filter element may comprise, for example, a feltor fabric imbibed or otherwise filled with the adsorbent component,either with or without a membrane filter layer. In a preferredembodiment, the filter system comprises a filter element of a felted orwoven backer material incorporating the mercury adsorbent component andlaminated to a membrane filter layer, most preferably comprisingexpanded PTFE. The backer material incorporating the adsorbent componentpreferably should have an air permeability Frazier Number greater than10, and more preferably greater than 20. In the filter element where theadsorbent component is laminated to a membrane filter layer, thecombined layers should have an air permeability Frazier Number greaterthan 1, preferably greater than 2. In a further embodiment, the filterincorporates additional functionality for removing or catalyzing otherspecies present in the gas stream, whether in a single backer layer orin multiple backer layers. One example of a filter material whichincludes added functionality is described in U.S. Pat. No. 5,620,669, toPlinke et al., wherein the filter comprises fibers of expandedpolytetrafluoroethylene (ePTFE) having a node and fibril structure, andcatalyst particles are tethered within the structure. Another example ofa structure incorporating additional functionality is one constructed inaccordance with PCT Publication No. PCT/US00/25776, described earlier,whereby an expanded PTFE membrane filter layer may be laminated to abacker imbibed with both the mercury adsorbent component and activecatalytic particles.

In a third embodiment of the invention, the filtration system comprisesa separate filter component incorporating the mercury removal adsorbentcomponent located downstream of the particulate filter element, such asin the case of a downstream filter array incorporating the binding agentand promoter. Suitable separate downstream filter arrays may includepanel or pleated filter constructions, cartridges, or other like.

Particular benefits of the present invention include providing a highefficiency of mercury removal during operation and collecting theparticulate and fly ash by the filter element upstream of the mercurycapture, thus reducing or eliminating contamination of the fly ash,which is a significant problem with carbon injection systems. Anotherbenefit includes the ability to easily and cost-effectively retrofitexisting filter systems for mercury control. A further benefit is thecapability of a single filtration system to provide multi-pollutantcontrol, including particulate, NOx, dioxins, furans, and mercurycapture in a single system.

BRIEF DESCRIPTION OF THE DRAWINGS

The operation of the present invention should become apparent from thefollowing description when considered in conjunction with theaccompanying drawings, in which:

FIG. 1 is a schematic picture of a conventional filter baghouseoperation;

FIG. 2 is perspective extended view of an embodiment of the presentinvention comprising a filter element incorporating an insert;

FIGS. 3A-E are perspective views of alternative adsorbent inserts of thepresent invention, and FIG. 3F is a partially expanded perspective viewof a further alternative adsorbent insert of the present invention;

FIG. 4 is a cut-away perspective view of an embodiment of an adsorbentfilter media of the present invention;

FIG. 5 is a perspective view of a partially separated section of anadsorbent filter media of the present invention;

FIG. 6 is a schematic picture of a filter system of the presentinvention; and

FIG. 7 is a perspective view of a test apparatus utilized in connectionwith the present invention.

DETAILED DESCRIPTION OF THE INVENTION

As noted, the present invention is an improved filtration system,filtering method and unique chemical composition for capturing mercuryand other pollutants in flue gases generated by process gas streams.

The improved filtration system may take various forms depending on thetype of filter system most desired for a particular application;however, the invention includes at least a filter element or elementsand an adsorbent component having a composition suitable for capturingmercury on the downstream side of the filter element.

Referring to FIG. 2, there is shown an exemplary embodiment of a filterelement 130 incorporating an adsorbent insert 133 in accordance with thepresent invention. Particularly, there is shown a filter bag 134supported by a filter bag cage 136 and an adsorbent insert 133 locatedinside the filter bag cage 136. In this embodiment, the insert 133 has asleeve or tubular geometry and is supported by an insert cage 138.Insert cage 138 is an optional component and is most preferably utilizedwhen a flexible insert is used. In the case of a construction where arelatively more rigid, self-supporting insert is provided, no insertcage would be necessary.

FIGS. 3A, 3B and 3C show an alternative construction comprising a paneladsorbent insert for a filter element, wherein the insert 133 comprisesa frame or housing 138 holding an adsorbent component media 140. Themedia 140 may be either flat as shown in FIG. 3A or may incorporatepleats 142, as shown in FIG. 3B. FIG. 3C shows a side perspective viewof the insert 133. One or more inserts may be positioned in or attachedto the filter element by any suitable means in a downstream orientation,as described above. FIGS. 3D and 3E show further hub-and-spoke andpleated geometries, respectively, for the suitable filter inserts 133which may be incorporated with the filter element in the presentinvention. FIG. 3F shoes a further alternate configuration of a suitableinsert 133 of the present invention, wherein the insert 133 comprises acanister filter configured for a pulse jet cleaning filtration system.Specifically, a canister housing 150 is provided for housing a coiled orotherwise configured adsorbent component media 152, and a pulse pipe 154runs through the canister housing 150 to supply pulsed air to clean theparticulates from the surface of the filter element (not shown). Aretainer 156 comprising wire screen is located on the top of the housing150, held on by nuts 160, to retain the media 152 within the housing 150while still permitting flue gas to flow through the canister. Duck billvalve 158 remains closed during filter operation of the filter system,but opens during cleaning to pass cleaning air through the pulse pipe154 and to the filter element.

Referring to FIG. 4, there is shown a section of an adsorbent filtermedia 170, which comprises an adsorbent component 172, such as feltedactivated carbon fibers or other air-permeable high surface area supportmaterial having binding agent and a promoter thereon, or alternatively asubstrate having activated carbon or other air-permeable high surfacearea support particles with binding agent and promoter thereon imbibedinto the structure of the substrate. Optional additional layers 174 arelocated on either or both sides, as shown, of the adsorbent The layers174 are attached to the adsorbent component by any suitable means, suchas stitching 176, as shown, or lamination, or the like, depending on therequirement of the filtration system. Such a filter media 170 may beformed into a flexible filter insert for location within a filterelement, or in an alternative embodiment, the filter media 170 may beformed into a filter element itself, such as a filter bag.

Referring to FIG. 5, there is shown a perspective view of a partiallyseparated section of an adsorbent filter media 170. The media comprisesan adsorbent component comprising, in this embodiment, a felted backermaterial 172 incorporating a binding agent and promoter and a membranelayer 178. This adsorbent filter media 170 is suitable for formationinto a filter element of the present invention, such as a filter bag,pleated filter cartridge, etc., wherein the membrane provides surfacefiltration of the particles in the flue gas stream and the backerprovides downstream mercury removal. While a variety of membrane layersmay be employed, it is particularly preferred to employ an expanded PTFEmembrane, such as that described in U.S. Pat. No. 3,953,566, to Gore,U.S. Pat. No. 5,476,589, to Bacino, and U.S. Pat. No. 5,814,405, thesubject matter of these patents being specifically incorporated byreference herein, due to its exceptional filtration properties. ExpandedPTFE (ePTFE) in the form of a membrane has a number of desirableproperties which makes it a particularly desirable filtration material.For example, ePTFE has many microscopic holes or “micropores”, such ason the order of 0.05 to 10 μm across, which allow fluid molecules topass through but restrict the passage of particulates, such as fine dustand the like. Additionally, the surface of an expanded PTFE membrane canbe readily cleaned of accumulated contaminants, vastly improving theoperative life of the filter. Other suitable nonwoven or woven backersmaterials may also be used. The membrane layer 178 may be attached tothe backer, either continuously or discontinuously, such as by means oflamination, welding, sewing, tacking, clamping, or other suitableattachment means.

Referring to FIG. 6, there is shown an exemplary schematic depiction ofa third embodiment of a filtration system 180 of the present invention,wherein an separate adsorbent filter array is located downstream of abaghouse. Particularly, filtration system 180 comprises a baghouse 182having gas flow inlet 183, filter elements 184 oriented in tube sheet186, and a pulse cleaning system comprising pulse pipe 188 and pulsevalve 189. An adsorbent filter array 190 incorporating adsorbent filtercomponents in the form of pleated panel filters 192 is orienteddownstream of the baghouse 182 to remove mercury from the flue gasstream before it passes out of the filter system through outlet 194.Alternative adsorbent filter arrays may be provided in the filtrationsystems of the present invention, provided they incorporate an adsorbentcomponent comprising an air-permeable high surface area support, abinding agent and a promoter. Moreover, the upstream particulate filtercomponent of the filtration system may comprise any conventionalparticulate filtration system, such as a baghouse, as described, anadvanced hybrid particulate collector system, such as that described inU.S. Pat. No. 5,938,818, to Miller, or any comparable particulatefiltration unit.

Test Methods

Bet Method for Surface Area Determination

The BET method, first reported by Brunauer, S., Emmett, P. H., andTeller, E. (1938), J. Amer. Chem. Soc. 60, 309, was used to determineapparent surface area from nitrogen adsorption isotherms. Isotherms weremeasured using a Coulter SA 3100 instrument (Coulter Corp., Miami, Fla.)after outgassing at 300° C. for 8 hours under dynamic vacuum unlessstated otherwise.

Mercury Capture Efficiency and Capacity Test

This test is used to measure the efficiency with which small samples oftextiles, needle-felted or non-woven fabrics, permeable tapes, and othertypes of permeable sheet goods used as filters can capture elementalmercury vapor from a hot, flowing gas stream as a function of time understandardized conditions. Total mercury capture (capacity, seedefinitions below) up to a designated efficiency level (see definitionsbelow) can also be determined. Although the reactor apparatus utilizedfor these measurements is capable of measuring performance with avariety of gas compositions, temperatures, flow rates, and end-pointefficiencies, we have selected a particular parameter set for testing.This test accelerates mercury loading with respect to loading ratesexperienced under typical field conditions and generates results thatcan be correlated with performance of the specimens exposed to fluegases in commercial facilities.

Essentially, a flowing gas stream at 300 cm³/min (NTP) consisting of 1(vol.) ppm of zerovalent mercury vapor 7 to 8 (vol.) percent watervapor, optionally a fraction of oxygen and other flue-gas components,and the balance nitrogen is preheated to 185° C. and passed through apre-weighed and dried 2.54 cm diameter disk of porous sample mountedorthogonally to the direction of flow of gas and held isothermally at185° C. such that gas is distributed uniformly radially across the planeof fabric and moved through the fabric in a plug-flow manner. Theincident mercury concentration is held constant throughout the run bythe mercury generation section of the apparatus using NIST traceablefixed-rate mercury permeation tubes held at constant temperature. Theconcentration of mercury in the emerging gas stream (outlet) isdetermined repeatedly (semi-continuously) with a pre-calibrated, on-linedetector after the gas stream is conditioned to remove water vapor, toreduce any oxidized mercury to the zero-valent state, and to dilute themercury to the concentration range of greatest analytical sensitivity ofthe detector. To enable measurement of the incident (inlet)concentration of mercury periodically, a piping pathway to bypass thetest specimen can be selected by appropriate valve configuration in theapparatus. Efficiency, defined below, is computed repeatedly during thetest corresponding to each point of time-on-stream of the specimen. Whenefficiency falls below a predetermined target, usually 40%, the test isterminated, the total time on-stream to that point determined, and thetotal mercury adsorbed (capacity, defined below) to that point computedby integrating the amount of mercury in the emerging stream over theentire run to the end point and subtracting this quantity from thecomputed steady-state incident cumulative challenge amount. The sampleis then removed, weighed, and optionally independently analyzed forspeciated mercury content by a destructive wet analytical method knownas the Ontario Hydro method, ASTM D22.03.01, described below. A separatealiquot of representative virgin fabric specimen is tested for airpermeability using a separate test known as the Frazier test, describedbelow. Mercury material balances throughout the reactor and particularlyaround the reactor analytical system and the mercury generation systemfrequently are corroborated against the Ontario Hydro method, describedbelow, using a 3 hour sampling time and the mini-impinger option of themethod. The equipment and piping path of the test apparatus is describedbelow.

Referring to FIG. 7, the mercury generation system 201 is based on aVICI Metronics 10 cm mercury permeation tube 202 which delivers 1980 mgHg/minute at 100° C. Optional pressure gauge 203 may be attached to aport of the manifold 204. Gases are passed from the manifold 204 to afore-section of the reactor 205 which is packed and heated to pre-mixand pre-heat the inlet gases to about 180° C., at 300 cm³/min, NTP. Thereactor holder (not shown) consists of a stainless steel outer housingwhich disassembles into two pieces of precisely machined and matchedflanged fittings housing a fluoropolymer clamping system that defines aprecisely cut 2.54 cm diameter circular disk of filter medium exposed tothe gas stream. The fluoropolymer holder seals to a lip cut in thestainless steel outer housing, which, in turn, screws together to make agas tight seal around the test piece. A matching punch cutter is used tocut a test piece of precise dimensions to fit the sample holder; the cutfilter test piece is placed in a constant humidity chamber then weighedprior to installation in the holder. A thermocouple (not shown) measuresthe filter temperature through a threaded side port. The inlet piping206 to the reactor 205 is heat traced and the reactor 205 itself ishoused in an isothermal oven 207 held at 185±1° C. A bypass line 208 isconstructed with valving (209 a, 209 b, 209 c) to isolate the reactor205 while still allowing inlet gas to flow to the detector 210. A valve211 in the line leading from the reactor 205 contains a sampling portfor optional Ontario-Hydro testing via an external glass train (notshown). (The valve 211 diverts the normal flow path downstream of thereactor when such Ontario-Hydro testing is carried out.) In the normalflow path for on-line mercury analysis, the effluent gas from thereactor 205 is channeled to a water removal system 213 that consists ofa countercurrent flow air dryer that contains a 10 foot Nafion™ tubefrom Permapure Company. Next in line is reduction furnace 212 held at920° C. through which the gas residence time is 2.5 seconds. Independenttests of the mercury reduction furnace and water removal system showedno hold up of and complete reduction of mercury. Effluent from thedrying system contains less than 0.1% water. After passage through acalibrated rotameter 214, dilution nitrogen is added to the stream, 1000cm³/min NTP, prior to the gas stream moving to the analyzer section 210.The analyzer 210 is a Buck Scientific Company Model 400A MercuryAnalyzer which utilizes ultraviolet detection at 254 nm through a quartzcell to detect elemental mercury. The span range of the analyzer outputis inspected and adjusted if necessary each time the reactor system istaken off-line. Data acquisition is accomplished using Strawberry Treesoftware and appropriate computer interface boards. All tubing or wettedparts in the system are either constructed of polytetrafluoroethylene,glass, or, minimally, stainless steel.

Ontario-Hydro Test

This test method is used to measure precise concentration of mercuryspecies in the gas stream in both elemental and oxidized forms and toanalyze mercury content of solid sorbents after HF digestion. The methodis ASTM Method D22.03.01 of the Task Group for Elemental and OxidizedMercury entitled “ASTM Standard Test Method for Elemental, Oxidised,Particle-Bound and Total Mercury in Flue Gas Generated from Coal-FiredStationary Sources. (The Ontario-Hydro Method)”, Z6590Z, adopted byD22.03.01 Task Group for Elemental and Oxidized Mercury, issued 2002, inpress. The mini-impinger (25 ml) option is followed to analyze gasstream from the reactor using a 3 hour sampling time. Analysis ofirreversibly trapped mercury by filter systems using the on-line method(not limited to 40% mercury capture efficiency) and the Ontario-Hydroanalysis of HF-digested samples generally agree to ±20%.

Definition of Terms

“MERCURY CAPTURE EFFICIENCY PERCENTAGE,” or “MCEP”, is defined as theinstantaneous volume fraction of incident (inlet) mercury that isremoved (adsorbed) by the test specimen under reaction conditionsdescribed above, expressed as a percent and inferred by the differencebetween inlet and outlet gas stream mercury concentrations:MCEP = {(100) × [(Instantaneous  inlet  gas  stream  mercury  concentration) − (Instantaneous  outlet  gas  stream  mercury  concentration)]/  [(Instantaneous  inlet  gas  stream  mercury  concentration)]}In a test in which fast digital electronic data gathering techniques areused throughout the run, it is convenient to gather MCEP data overfinite, short time intervals during which the change in outlet mercuryconcentration with time can be considered negligible.

Mercury capture efficiency can be normalized to thickness of the mercuryfilter test piece or to pressure drop or resistance to axial gas flowthrough the filter as measured by a standard test conducted on aseparate test piece of similar fabric or filter composition.

“MERCURY CAPTURE CAPACITY TO 40% EFFICIENCY”, or “MCC40”, expressed inmg Hg/g sample, is defined as the total accumulated milligrams ofmercury adsorbed per pre-test, humidity-equilibrated grams of sampleweight, by a test specimen held under the test conditions describedabove, from the beginning of the test until MCEP decays to 40%.

In a test in which digital electronic data gathering techniques are usedsuch that data are gathered within an unbroken stream of short timeperiods with negligible dead time between measuring intervals, i.e.,continuous batching of sampling intervals techniques, integration tocompute accumulated mercury adsorbed over the test period may besimulated by finite summations${MCC40} = {\sum\limits_{{MCEP} = {{initial}\quad\%}}^{{MCEP} = {40\%}}\quad A}$Where  A = {[(Incremental  steady-state  inlet  Hg  challenge, expressed  as  mg  of    Hg/second) × (Seconds  duration  of  sampling  interval)] −   [(Incremental  outlet  Hg  amount, expressed  as  mg  of  Hg/second) × (Seconds  duration  of  sampling  interval)]}/(grams, pre-test  humidity − equilibrated  weight  of  sample)]Frazier Test

The standard test for resistance to axial gas flow is the Frazier airflow test, which reports flow in cubic feet per minute per square footof material.

Air permeability was measured by clamping a test sample in a gasketedflanged fixture which provided in circular area of approximately 6square inches (2.75 inches diameter) for air flow measurement. Theupstream side of the sample fixture was connected to a flow meter inline with a source of dry compressed air. The downstream side of thesample fixture was open to the atmosphere.

Testing was accomplished by applying a pressure of 0.5 inches of waterto the upstream side of the sample and recording the flow rate of theair passing through the in-line flowmeter (a ball-float rotameter).

Results are reported in terms of Frazier Number, which is air flow incubic feet/minute/square foot of sample at 0.5 inches water pressure.

Without intending to limit the scope of the present invention, thefollowing examples illustrate how the present invention may be made andused: TABLE 1 High Temperature Mercury Capture Capacity and EfficiencyTest Results from Examples 1-11 Dynamic Mercury Dynamic Mercury TrappingTest Trapping Test Results, Temperature (° C.), MCC40 mass % Bindingmass % Promoter, additional gas (mg Hg/g untreated Example SubstrateAgent dehydrated components test piece) 2 (Control) ACF 0 0 185 0.27 1ACF 40.7% KI 11.2% Zn(CH₃CO₂)₂ 185 36.6 185, O₂ 46.2 3 ACF 48.6% KI 0185 25.8 4 ACF 0 16.8% Zn(CH₃CO₂)₂ 185, O₂ 0.04 5 ACF 48.6% KI   3%Zn(CH₃CO₂)₂ 185 30.1 185, O₂ 30.0 6 ACF 38.4% KBr 14.8% Zn(CH₃CO₂)₂ 185,O₂ 6.2 7 ACF 20.2% KBr 56.2% K₄P₂O₇ 185, O₂ 2.9 10 C-filled ePTFE 19.8%KI 0 185 8.6 felt 11 (Control) C-filled 0 0 185 0.002 ePTFE felt

EXAMPLE 1

A 4 inch by 6 inch section of VAF-90 activated carbon felt (ACF) was cutfrom a large sheet of material obtained from Shanghai No. 1 ACF CompanyCo., Ltd. (Shanghai, China) and weighed precisely. As received, thematerial had a nominal mass-per-unit-geometric-area of 260 g m⁻² and ameasured Frazier Number air permeability of about 60-65. The cut samplewas placed on a platen heated to about 90° C., slightly dampened with2-propanol, and impregnated using an aerosol sprayer under atmosphericconditions. The sample was treated on both sides until wet through butnot dripping with an aqueous solution of 1.0 F potassium iodide and 0.27F zinc acetate dihydrate. The wet sample was placed on a stainless steelscreen in a solvent-vented muffle furnace held at 200° C. for about onehour, cooled and equilibrated for 2 hours, then weighed. The driedsample displayed an air permeability of about 52-60 Frazier. A smallportion was sent to Galbraith Laboratories, Inc. in Knoxville, Tenn.,USA for chemical analysis of percent iodine, zinc, and loss on drying.Another portion of the sample was tested by a thermogravimetric methodfor weight loss on heating in air at 20° C./minute to 250° C. and heldfor 4 hours. This and similar samples were examined by opticalmicroscopy and by spectroscopic methods. A portion of the sample wastested using the High Temperature Mercury Adsorption Capacity andEfficiency Test, as described infra, using a reactor facility at the USEnvironmental Protection Agency Laboratories in North Carolina, USA.Results from this test and corresponding analyses are listed in Table 1.

EXAMPLE 2 Comparative

A sample of ACF felt was wet with distilled water, dried, and treatedsimilarly to the material described in Example 1 except for theimpregnation step. This material was subjected to the high temperaturemercury adsorption capacity and efficiency test as a control sample.Results from this test are listed in Table 1.

EXAMPLE 3

A sample was prepared similarly to the procedure of Example 1 exceptthat no zinc and a slightly higher loading of potassium iodide wereadded. Composition and results of testing in the High TemperatureMercury Adsorption Capacity and Efficiency Test appear in Table 1.

EXAMPLE 4

A sample was prepared similarly to the procedure of Example 1 exceptthat no potassium iodide was added and a higher zinc loading wasachieved. This sample was tested in the high temperature mercuryadsorption capacity and efficiency test. Results appear in Table 1.

EXAMPLE 5

A sample was prepared similarly to the procedure of Example 1 exceptthat a higher potassium iodide loading and a lower zinc acetate loadingwere added. This sample was tested in the high temperature mercuryadsorption capacity and efficiency test. Results appear in Table 1.

EXAMPLE 6

A sample was prepared similarly to the procedure of Example 1 exceptthat potassium bromide was used in place of potassium iodide and that aslightly higher zinc acetate loading was added. This sample was testedin the high temperature mercury adsorption capacity and efficiency test.Results appear in Table 1.

EXAMPLE 7

A sample was prepared similarly to the procedure of Example 6 exceptthat a lower loading of potassium bromide was added and potassiumpyrophosphate was used in place of zinc acetate at a substantiallyhigher loading. Test results using this sample in the High TemperatureMercury Adsorption Capacity and Efficiency Test are reported in Table 1.

EXAMPLE 8

A sample was prepared by the procedure of Example 1 using a square 1.5meter×1.5 meter piece of VAF-110 ACF in place of VAF-90 ACF. The VAF-110displayed a nominal mass-per-unit-geometric-area of 380 gm⁻². The driedsample of impregnated ACF was then sewn to the felt side of a layer ofRemedia™ Catalytic Filter Membrane-Felt Laminated composite produced byW.L. Gore and Associates, Inc. (Newark, Del.) then subjected tocatalytic efficiency testing and to Frazier air permeabilitymeasurements.

EXAMPLE 9

A large sample of ACF from Shanxi Xinshidai Import & Export Corporation,(Taiyuan, Shanxi, China) Import Company that was 5 millimeters thick anddisplayed a BET surface area of 1123 m².g⁻¹ was dampened with 2-propanolthen impregnated to incipient wetness with an aqueous solution of hotzinc acetate. The sample was partially air dried then heat treated at220° C. for about 17 hours. After cooling, the sample was impregnatedwith aqueous potassium iodide solution to the point of dripping thenrolled with a plastic cylinder to remove excess moisture. After dryingat 110° C., the sample was further heat treated overnight at about 200°C., cooled, then portions subjected to various tests. Among these wasexposure to elemental mercury vapor and air in a sealedpolytetrafluoroethylene-lined autoclave held at 190° C. for 9 hoursunder autogeneous pressure followed by cooling under mercury vapor. Themercury-treated sample was handled aerobically and transferred to atemperature-programmed-oxidation apparatus that had been interfaced witha quadrupole mass spectrometer as a detector. No significant emission ofmethylmercury above the parts-per-million range detection level wasobserved as the sample was heated in a mercury-free stream of helium andmonitored by mass spectrometry.

EXAMPLE 10

Activated carbon-filled PTFE fibers were prepared as described in U.S.Pat. No. 5,620,669 with the replacement of powdered catalyst with asufficient weight of activated carbon powder to result in a finalloading in the range of 40-46 percent by weight carbon. The activatedcarbon used was a coconut based carbon obtainable from Barnaby SutcliffCompany, milled to an average particle size of about 1 micron or finer.Resulting composite fibers averaged 0.51 g/denier with a tenacityaverage of 28,000 denier. These low tenacity fibers were chopped toeither 1.5 inch or 2 inch lengths, air blown, and mechanically cardedusing a multi-roller pilot-scale machine, then needled into a PTFE scrimfrom both sides. The resulting structure was a uniform felt of 25.8ounces-per-square-yard that displayed a permeability of 39 Frazier. Theformed felt was heat-treated at 200° C. for 30 minutes, without use of apin frame, to preshrink and tighten the structure. The heat finishedfelt displayed an air permeability of 14 Frazier and a BET surface areaof 99 m²g⁻¹ using nitrogen adsorbate. A portion of this felt waspre-moistened with 2-propanol then impregnated with an aqueous solutionof potassium iodide followed by drying at 110° C. and heat treatment at190° C. Results of the High Temperature Mercury Adsorption Capacity andEfficiency Test are reported in Table 1 along with potassium iodideloading.

EXAMPLE 11 Comparative

A portion of the activated-carbon-filled ePTFE felt prepared in Example10 without potassium iodide treatment was used as a control in the HighTemperature Mercury Adsorption Capacity and Efficiency Test. Results arereported in Table 1.

EXAMPLE 12

A mercury trapping filter insert was prepared by combining two layers of1.4 oz/yd² woven e-glass, style 1080 from JPS Glass, (Slater, S.C.) witha middle layer of VAF 90 activated carbon felt from Shanghai No. 1 ACFCo., Ltd, (Shanghai, China). The three 42 inch by 180 inch layers werequilted together on a long-arm quilting machine, from Gammill QuiltingMachine Company (West Plains, Mo.), using 1200 DENIER RASTEX@ sewingthread from W. L. Gore & Associates, Inc. (Elkton, Md.). The layers werestitched together in horizontal and vertical lines, spaced 3 inchesapart to form 3 inch squares. The quilted filter insert was cut into asmaller sample measuring 13.25 inches by 36 inches. This piece then wastreated in a shallow tray by pouring onto it a hot solution of 1.0 Fpotassium iodide and 0.27 F zinc acetate dihydrate to incipient wetness.The lightly-moistened mat was hung in a large air-circulated oven atapproximately 200-260° C. overnight. After drying, it was formed into a3.8 inch diameter×34 inch tube and sewn with a triple stitch using thesame type of 1200 denier thread as was used for the quilting process.The dried sample contained 21.3% by mass potassium iodide and 8.2% zincacetate, (including the mass of the fiberglass reinforcement).

The filter insert sleeve was pulled over a perforated spiral formedmetal tube made from 304 stainless steel, supplied by Arcor Inc.(Chicago, Ill.). The tube material was 0.014 inches thick with anoutside diameter of 3.4 inches. The perforated hole pattern was0.125×0.188 inches, staggered, with an open area of 33%. The filterinsert, supported by the metal tube was then inserted into a 10 wire,carbon steel filter bag cage with an outside diameter of 4.5 inches,supplied by Royal Wire Products, Inc. (North Royalton, Ohio). The entireassembly was then inserted into a 34 inch long filter bag, constructedfrom 16 oz./yd² polyphenylene sulfide needlefelt, laminated withGORE-TEX® membrane from W.L. Gore & Associates, Inc. (Elkton, Md.).

EXAMPLE 13

A 16 foot length of 17 inch wide ACF mat weighing 710 grams was unfurledonto a shallow trough molded from aluminum foil then wet thoroughly withabout 7 liters of a hot solution of 0.7 F potassium iodide and 0.15 Fzinc acetate dihydrate. The wetted mat was formed into a lengthwise 4′!! inch diameter cylinder with approximately 3 inch wall overlap arounda stainless-steel wire support cage used as a form. The aluminum foil ofthe trough then was tightly wrapped over the formed tube to hold the matin the cylindrical shape while drying. Holes were poked into the foilwrapping to allow for moisture release and the form-fitted mat placedinto a large air-circulated oven at approximately 200-260° C. overnight.The stiffened, dry prototype was carefully unwrapped and removed fromthe wire cage form. A small piece was cut for mercury capture capacitytesting from the end of the cylinder and the rest inserted into an 18foot PVC pipe. The pipe containing the ACF mat cylinder, in turn, wasinserted into a wire cage/filter bag assembly, and the pipe carefullywithdrawn, leaving behind the free-standing ACF insert within the filterbag internal cage. The entire membrane bag/cage/insert assembly wasinstalled into a pulse test apparatus and fitted with accelerometers andflow and pressure transducers along its length. Simulated pulsecleaning, pressure drop, and durability tests were then performed on theprototype assembly. Test results were compared to those derived usingthe filter bag assembly without the ACF mercury-trapping insert.

EXAMPLE 14

A 17 inch by 120 inch section of VAF-90 activated carbon felt (ACF) wascut from a large sheet of the material obtained from Shanghai No. 1 ACFCompany Co., Ltd. (Shanghai, China). The sample was treated with a hotsolution of 0.7 F potassium iodide and 0.15 F zinc acetate dihydrate,according to the procedure described in Example 13. The wetted mat wasfolded into 5 pleats that were 12″ deep. The treated sample was placedinto a large air-circulated oven at approximately 200-260° C. to dryovernight. The stiffened, pleated prototype was removed from the ovenand inserted into a pleated support structure made from 20 gauge, Type304 Stainless Steel from Spantek/Coastal Division of U.M. 1,(Summer-field, NC). Gravimetric binder and promoter loading computed forthe dried sample was 40.7 mass % potassium iodide and 11.2 mass % zincacetate, anhydrous basis.

While particular embodiments of the present invention have beenillustrated and described herein, the present invention should not belimited to such illustrations and descriptions. It should be apparentthat changes and modifications may be incorporated and embodied as partof the present invention within the scope of the following claims.

1. A filter system comprising at least one particulate collection filterelement; and an insert oriented on the downstream side of said at leastone particulate collection filter element, said insert comprising a highsurface area support material having on at least a portion thereof acoating composition comprising a binding agent and a promoter, wherebysaid composition is capable of chemical reaction with elemental andoxidized mercury to bind said elemental and oxidized mercury.
 2. Afilter system comprising at least one particulate collection filterelement selected from a filter bag and a filter cartridge; and aremovable insert oriented within said at least one particulatecollection filter element, said insert comprising a high surface areasupport material having on at least a portion thereof a coatingcomposition comprising a binding agent and a promoter, whereby saidcomposition is capable of chemical reaction with elemental and oxidizedmercury to bind said elemental and oxidized mercury.
 3. The filtersystem of claim 2, wherein said filter element comprises a depthfiltration element.
 4. The filtration system of claim 2, wherein saidfilter element comprises a surface filtration element.
 5. The filtersystem of claim 2, wherein said filter element comprises an expandedPTFE membrane.
 6. The filter system of claim 2, wherein said coating onsaid air permeable high surface area support material comprises abinding agent comprising potassium iodide and a promoter comprising zincacetate.
 7. A filter system comprising at least one particulatecollection filter element; and a mercury collecting filter elementoriented separate from and downstream of said particulate collectionfilter element, said mercury collecting filter element comprising a highsurface area support material having on at least a portion thereof acoating composition comprising a binding agent and a promoter, wherebysaid composition is capable of chemical reaction with elemental andoxidized mercury to bind said elemental and oxidized mercury.
 8. Thefilter system of claim 7, wherein said mercury collection filter elementis in the form of a panel filter array.
 9. A filter element comprisingan air permeable high surface area support material having on at least aportion thereof a coating composition comprising a binding agent and apromoter, whereby said composition is capable of chemical reaction withelemental and oxidized mercury to bind said elemental and oxidizedmercury; and a sheet of microporous membrane mounted on at least oneside of the porous support material, the sheet having a pore structuresufficiently small enough to serve as a barrier to isolate particles inthe fluid stream away from the porous support material.
 10. A method ofseparating particulates and heavy metals from a gas stream comprising:providing a filter system comprising at least one particulate collectionfilter element; and a mercury collecting filter element orienteddownstream of said particulate collection filter element, said mercurycollecting filter element comprising an air permeable high surface areasupport material having on at least a portion thereof a coatingcomposition comprising a binding agent and a promoter, whereby saidcomposition is capable of chemical reaction with elemental and oxidizedmercury to bind said elemental and oxidized mercury; placing said filtersystem in a gas stream with the particulate collection filter elementoriented in an upstream position relative to the mercury collectingfilter element to collect the particles suspended in the gas stream;whereby said particulate is collected on said particulate filter elementand said elemental and oxidized mercury is collected downstream of saidparticulate filter element by said mercury collecting filter element.